The present invention relates generally to indirectly mounting a pressure sensitive silicon chip to a metallic header such as stainless steel and more particularly to the use of a glass tube holder for the silicon chip and a iron-nickel (Kovar) interface between the glass tube and the header.
In electronic pressure transmitters an existing problem is the mounting of the silicon chip to the stainless steel header. The very different thermal expansion coefficients of the silicon chip and the stainless steel would cause the silicon chip to be damaged due to expansion and contraction of the stainless steel if it was directly bonded to the stainless steel. Therefore a holder in the form of a glass tube is often used for mounting the silicon chip. When a glass tube holder is used to mount the silicon chip, the problem becomes how to connect the glass tube to the stainless steel.
Various methods of indirectly mounting a glass tube to a steel header have been used in the past. In particular U.S. Pat. No. 4,019,388 issued to George R. Hall et al. on Apr. 26, 1977 discloses a glass tube holder for a silicon chip and a nickel-iron holder for indirectly mounting the glass tube to a steel housing. The Hall patent recognizes the advantage of using a nickel-iron or Kovar interface which may be joined to both glass and stainless steel and has a coefficient of thermal expansion that is intermediate between the thermal expansion coefficient of the silicon chip and that of the stainless steel.
However, the Hall patent does not disclose the considerations for releasing stress on the glass tube through a particular Kovar interface construction and through a particular configuration for connecting the interface to the stainless steel.
Thus a need exists for a Kovar interface that is both constructed in a way and connected to the stainless steel in a way to reduce stress on the glass tube.